This invention relates to the transfer of nuclear fuel assemblies and in particular to a method and apparatus for transferring fuel bundles from the nuclear reactor pool inside the containment building to the spent fuel storage pool external to the containment building.
In refueling a nuclear reactor, the radioactive fuel bundles must be removed from the reactor core and stored for an extended period of time in a spent fuel pool. In order to avoid the hazards due to radiation, the nuclear reactor core is flooded with water to a substantial depth above the top of the core with the fuel elements removed under water. Since these fuel elements are highly radioactive and still produce heat, known as decay heat, for a period of several months, they cannot be immediately removed from the plant but must be stored preferably under water which provides radiation protection and the necessary cooling.
When radioactive isotopes of these spent fuel elements have decayed sufficiently, they may be removed and shipped for reprocessing. Since the nuclear reactor will be back in operation at this time, it is preferable that the spent pool be located outside of the reactor containment building. Since the reactor containment building is designed to withstand relatively high pressures and to provide radiation shielding, large openings in its walls are cumbersome and expensive. The elongated fuel elements are therefore longitudinally passed through openings in the reactor containment.
Accordingly, the spent fuel pool is generally located adjacent to the reactor pool but outside of the containment building. A transfer tube capable of being closed by means of gate valves, and/or blind flanges joins the two pools. Transfer of fuel between the two pools requires that the fuel be placed into a fuel carrier mounted on a fuel transfer carriage. The fuel carrier is rotated to a horizontal position and the fuel carriage with fuel carrier and fuel assembly is translated through the horizontal tube into the spent fuel storage pool. The fuel carrier and fuel assembly is then returned to the vertical position, and the fuel assembly is lifted out of the fuel carrier and placed in storage racks within the spent fuel pool.
If a relatively short transfer tube is used to connect the two pools, a transfer system can be used which is driven in either direction by a single device located near either pool. When the length of the transfer tube is substantially lengthened, the requirements for the size of the pool on the driven end become inordinately large due to the requirement that the carriage be of increased length. It, therefore, becomes infeasible to drive the transfer system from one side so that two drives are required. An alternative to the two drive approach is the installation of additional equipment in or through the tunnel in preparing to transfer fuel. This equipment can not be left in place since the fuel transfer tube must be closed during normal operation of the reactor.